Recording systems for visualizing image information through an electrostatic latent image such as electrophotography are now widespread in various fields. In electrophotography, an electrophotographic photoreceptor is charged and then exposed to light to form an electrostatic latent image, the latent image is developed with a developer containing a toner, and the toner image is transferred and fixed to visualize the image. The developer used herein includes a two-component developer comprising a toner and a carrier, and a one-component developer comprising a toner, e.g., a magnetic toner alone. In the two-component developer, since a carrier bears such functions as agitation, delivery and charging of the developer, the two-component developer is characterized by satisfactory controllability and largely employed for the present time. In particular, developers using a resin-coated carrier are excellent in charge controllability and are relatively easy to improve environmental dependence and stability with time.
Development was formerly carried out by cascade development, but nowadays magnetic brush development using a magnetic roll as a developer carrier is dominant.
Magnetic brush development using a two-component developer has such disadvantages as reduction of image density and considerable background stain both due to reduction in charging properties of the developer, reduced image quality and consumption loss of the carrier both due to adhesion of the carrier onto the image, and occurrence of unevenness of image density. It is considered that the mechanism of adhesion of the carrier onto the image is such that with a reduction in resistance of the carrier, the induced charges are injected into the image areas, resulting in adhesion of the carrier to the image areas; or it is considered that charge quantity of the carrier after development becomes excessive on account of insufficient control of the upper limit of charge quantity of the carrier, resulting in adhesion of the carrier to the edges of the image areas.
In recent years, negatively chargeable organic photoreceptors have been spread, and reversal development in which an electrostatic latent image is formed by using laser, etc. has been frequently applied to inorganic photoreceptors. Accordingly, there has been increasing demands for high quality developers using a positively chargeable toner. Accordingly, it has been examined that various charge control agents are added to toners to stabilize the charge quantities of the toners at a proper value of positive charge, and further it has been extensively examined that the charging of the toners is controlled by choosing a resin with which the carrier is coated.
Quaternary ammonium salts are known as colorless high-safety charge control agents [see, JP-A-49-51951 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")]. However, when the quaternary ammonium salt is used in the two-component developer using a positively chargeable toner, charge quantity is insufficient merely by adding a quaternary ammonium salt type charge control agent to a toner, and hence it is necessary to coat a carrier with a negatively chargeable resin.
In particular, fluorine-containing resins have been extensively studied as negatively chargeable resins for use in coating the carrier in recent years. However, there are problems that the fluorine-containing resins have poor adhesion to core particles and the coated layer thereof has poor abrasion resistance.
In order to improve adhesion of the fluorine containing resins to core particles, it has been proposed to coat the core particles with a mixture of a fluorine-containing resin and the second resin well compatible with the fluorine-containing resin, such as a methyl methacrylate copolymer as disclosed in JP-A-54-110839 and JP-A-56-113146. However, such a mixture system adversely affects such excellent properties as solid lubricating properties and low tackiness inherent in the fluorine-containing resins, resulting in interference with an improvement of the surface stain resistance of the carrier.
Further, a developer comprising a toner containing a quaternary ammonium salt type charge control agent and a carrier having a vinylidene fluoride copolymer-coated layer has been proposed as disclosed in JP-A-1-204073. However, the above-mentioned problems can not be solved, and it is difficult to freely control the charge quantity of the toner.